This application seeks to challenge the research paradigm that the spike output of the eye consists of invariant (all-or-none) signals. The proposed experiments will test the hypothesis that, in addition to lowering the light sensitivity o photoreceptors, daylight modulates the shape and number of spikes in the adult mammalian optic nerve. The long-term objectives of this work are (i) to identify properties of optic nerve spikes that are altered by illumination which light-adapts photoreceptors, increases endogenous dopamine release, and activates a calcium/calmodulin-dependent protein kinase (CaMKII) in retinal ganglion cell (RGC) somata and axons, and (ii) to distinguish changes in spiking that result from dopamine receptor activation versus those that result from light-induced repetitive spiking. These goals will be pursued in three specific aims. Aim 1 tests whether light, dopamine, and repetitive spiking alter spike shape, duration, and threshold. Although dopamine and repetitive spiking alter spiking in RGCs via various signaling components, new preliminary data presented in this application show that illumination recruits one of these - CaMKII - in RGC axons. Therefore, Aim 2 tests whether dopamine and repetitive spiking activate CaMKII, and whether CaMKII inhibition blocks effects of illumination, dopamine, and repetitive spiking on spikes. Aim 3 will gauge the impact of dopamine receptor activation and repetitive spiking on the spike output of the eye by measuring the responses of RGCs to a RGC-specific dopamine receptor agonist, electrode-stimulated spiking, and light stimuli that identify various functional cell types. These experiments will utilize two novel approaches to identify effects of light adaptation on RGC spiking. First, multi-electrode arrays (MEAs) will be used to initiate spikes in RGCs, and elicited spikes will be recorded in the nerve fiber layer with MEA electrodes and in the optic nerve with suction electrodes. These spikes will be compared before and during light adaptation, and before and after electrode-induced repetitive spiking. Receptor antagonists and kinase inhibitors will be applied before or during these protocols to test involvement of dopamine receptor and CaMKII activation. Secondly, a structural analog of dopamine (SKF-83959) that activates receptors unique to RGCs will be applied to dark-adapted retinae to test whether dopamine mimics the effects of light adaptation on spikes. Effects of brief and long exposures to SKF-83959 and light will be compared, because dopamine is thought to be released for hours during normal daylight. The expected outcomes of this work will identify novel components of light adaptation in the retina, novel effects of dopamine and CaMKII activation on retinal electrophysiology, and a novel contribution of RGC axons to spikes sent by the eye to the brain. These results will fundamentally advance our understanding of how state- and activity-dependent modulation contributes to light adaptation, and of the signals and mechanisms that the retina uses to report changes in illumination to higher levels of the visual system.